Optical fiber manufacture

ABSTRACT

The specification describes methods for the manufacture of very large optical fiber preforms wherein the core material is produced by MCVD. Previous limitations on preform size inherent in having the MCVD starting tube as part of the preform process are eliminated by removing the MCVD starting tube material from the collapsed MCVD rod by etching or mechanical grinding. Doped overcladding tubes are used to provide the outer segments of the refractive index profile thus making most effective use of the MCVD produced glass and allowing the production of significantly larger MCVD preforms than previously possible.

RELATED APPLICATION

This application is a continuation of application Ser. No. 10/366,888,filed Feb. 14, 2003.

FIELD OF THE INVENTION

This invention relates to optical fiber manufacture, and morespecifically to improved optical fiber preform fabrication techniques.

BACKGROUND OF THE INVENTION

Manufacture of optical fiber performs, the glass blanks from whichoptical fibers are drawn, typically involves a rotating lathe, wherepure glass or glass soot is formed on a rotating member by chemicalvapor deposition or a modification thereof. All successful methods offiber manufacture should assure that the optical quality and purity ofthe preform glass is high. In particular, the glass making up thecentral portion or core of the preform should be of the highest puritysince most of the optical power in the fiber will be carried within thisregion. A significant advance in this direction occurred with theintroduction of the so-called Modified Chemical Vapor Deposition (MCVD)process in which the glass-forming precursors are introduced into arotating hollow starting tube, and glass material is deposited on theinside wall of the hollow tube. The better control over the reactionenvironment provided by this inside deposition process, allowedexceptionally pure glass to be produced in the critical core region.

The MCVD technique has evolved to a highly sophisticated manufacturingtechnique, and is widely used in commercial practice today. However,limiting aspects in MCVD and similar inside deposition processes are thesize and quality of the starting tube and the total amount of glass thatcan be deposited inside a starting tube. The limitation on the totalamount of deposited glass necessarily limits the number of distinctdoped regions or segments of a given size that can be accommodated in apreform of this type.

Another preform fabrication technique, Vapor Axial Deposition (VAD), wasdeveloped in which the CVD-formed silica soot deposits and grows axiallyfrom a starting mandrel. In a subsequent manufacturing stage or stages,the soot body is purified, dried and sintered into pure glass. At somepoint, the mandrel is separated from the deposited body and the entirepreform, unlike MCVD, may thus be made of CVD-deposited material. As ageneral proposition, VAD methods are effective and widely practiced, butthey still do not match the ability of MCVD to control precisely theradial deposition profile of index changing dopants such as germaniumand fluorine. Because of this, VAD methods and other sootdeposition/subsequent sintering methods such as Outside Vapor Deposition(OVD) are limited in the complexity of the fiber designs that can beefficiently produced.

Considering that in a single mode optical fiber the core and innercladding together carry greater than 95% of the optical power buttypically comprise less than 5% of the fiber mass, all manufacturingprocesses focus special attention on the fabrication of this region.This has resulted in approaches to preform manufacture, where the coreand inner cladding regions of the preform are produced by a relativelyadvanced and expensive method, while the outer cladding, the bulk of thepreform, may be produced by a less demanding, and less expensiveprocess. The integration of the core rod and the cladding is carried outin an overcladding process. The overcladding process in general isdescribed for example in U.S. Pat. No. 6,105,396 (Glodis et al), andPCT/EPT00/02651 (25 Mar. 2000), which are incorporated herein byreference for details of the general techniques. The overcladdingprocess may consist of multiple steps, each adding a distinct claddingregion, if this is required by the complexity of the desired fiberrefractive index profile. The most prevalent process of this type is theso-called rod-in-tube method, where the core rod is made by a very highquality dopant-versatile process, and the cladding tube is often made ofless expensive, lower purity or single composition glass. In therod-in-tube overcladding process, the core rod is inserted into thecladding tube, and the tube collapsed around the rod to form a unitarybody. Again, multiple overcladding steps may be used and in some casesone or more of the final overcladding processes may be combined with thefiber drawing operation.

State of the art manufacture for very large preforms now makes use ofcore rods produced by Outside Vapor Deposition or Vapor AxialDeposition. If a tube overcladding process is used, suitable claddingtubes may be produced by soot deposition or extrusion of fused quartz.Making these very large cladding bodies with a soot based syntheticglass process leads to high quality glass but requires extensiveprocessing and is relatively expensive. Large bodies of fused quartz areless expensive but are generally not of sufficient purity for largepreforms. A more economical approach for making high quality claddingtubes is to use sol-gel techniques. This well-known procedure isdescribed, for example, in J. Zarzycki, “The Gel-Glass Process”, pp.203-31 in Glass: Current Issues, A. F. Wright and J. Dupois, eds.,Martinus Nijoff, Boston, Mass. (1985). Sol-gel techniques are regardedas potentially less costly than other known preform fabricationprocedures. Options for producing the cladding tubes are addressed herefor completeness, but the focus of this invention is on the core rod.The term core rod is used for convenience since the core rod alwayscontains the central core material. However, the rod may comprise innercladding, or both inner and outer cladding, as well as the central core.These options will be described in more detail below.

For producing very high quality central core and inner claddingmaterial, the MCVD process would appear ideal. However, the MCVDstarting tube can be a limiting factor in several ways. The most directlimitation is when the glass in the MCVD starting tube is simply not ofsufficient quality and low loss for large state of the art preforms(where some fraction of the optical power would be carried by thestarting tube material). If the initial tube quality limitation isavoided by the use of ultra pure (and typically expensive) material tofabricate the starting tube, the exposure of the tube to theoxy-hydrogen torch typically used in MCVD as a heat source cancompromise the effective starting tube quality by the addition ofhydroxyl ions to a significant depth. Finally, the desired refractiveindex profile may require a dopant level in the region provided by thestarting tube glass that is not compatible with successful MCVDprocessing (viscosity, tube stability or heat transfer considerations).

It should be evident from the discussion above that the production ofvery large core rods for rod in tube methods appears to be most suitablyaccomplished by VAD or OVD type methods. While the MCVD process iscapable, along with the VAD and OVD processes, of producing very highquality glass, the MCVD glass is deposited inside a starting tube which,because of the reasons outlined above, can disadvantageously limit theapplication of the rod in tube method to preforms below a given size.

SUMMARY OF THE INVENTION

We have developed a technique that allows the use of MCVD for producinglarge preform core rods in a rod-in-tube process. High-quality glass isdeposited on the inside of a MCVD starting tube, and the tube collapsedin the usual manner to form a solid rod. The starting tube, at thispoint the outside shell of the rod, is then removed from the solid rodleaving just MCVD-deposited material. The rod is then inserted into acladding tube and the cladding tube collapsed to form the preform. Thepreform, following this method, has a core region consisting entirely ofMCVD deposited material. Optionally, one or more inner cladding segmentsmay be deposited along with the central core during the MCVD process,and the preform completed by the application of one or more claddinglayers over the MCVD central core and inner cladding layers.

In a preferred embodiment, the overcladding operation is accomplished bycontrolling the atmosphere in the gap between the MCVD rod and theovercladding tube in much the same way that the original MCVD processcarries out the glass forming reaction inside a tube to isolate theglass forming reaction from the environment.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1 and 2 are schematic representations of a rod-in-tube process forthe manufacture of optical fiber preforms;

FIG. 3 is a schematic representation of an MCVD process showingdeposition of high purity glass on the interior walls of the MCVDstarting tube;

FIGS. 4 and 5 are schematic diagrams of preform profiles that are amongthose adapted to be produced by the method of the invention;

FIG. 6 represents the collapse step in the MCVD process;

FIG. 7 represents the step in the inventive process of removing the MCVDstarting tube, leaving a core rod adapted for a rod-in-tube method;

FIG. 8 represents the step of collapsing an overclad tube onto the MCVDglass rod, using a modified MCVD apparatus or a dedicated overcladdinglathe, which provides isolation from the ambient atmosphere and where asuitable drying/etching gas or gases can be made to flow in the innergap during the process;

FIG. 9 is a schematic representation of a fiber drawing apparatus usefulfor drawing preforms made by the invention into continuous lengths ofoptical fiber;

FIG. 10 is a refractive index profile for a typical optical fiber madeaccording to the invention; and

FIG. 11 is a plot of optical loss (dB/km) vs wavelength (nm) for theoptical fiber used for the data of FIG. 10.

DETAILED DESCRIPTION

Typical rod-in-tube methods are described in conjunction with FIGS. 1and 2. It should be understood that the figures referred to are notnecessarily drawn to scale. A cladding tube representative of dimensionsactually used commercially has a typical length to diameter ratio of10-15. The core rod 12 is shown being inserted into the cladding tube11. There exist several common options for the composition of the corerod, It may be pure silica, adapted to be inserted into a down-dopedcladding tube. It may have a pure silica center region with a down-dopedinner cladding region. It may have an up-doped, e.g. germania doped,core region surrounded by a pure silica cladding region. It may have anup-doped center core region surrounded by a down-doped inner claddingregion. All of these options, and many variations and elaborations, arewell known in the art and require no further exposition here. However,the preferred embodiment of the invention is aimed at the case where thecore rod has either a central core region (only) or a central coreregion and one cladding region with the remaining cladding regions orsegments provided by the overcladding process. This takes full advantageof the MCVD process and allows a substantial increase in preform sizerelative to prior art. A typical profile has a central core of up-dopedmaterial, typically germanium doped, and at least one un-doped,neutrally-doped or down-doped region adjacent to the germanium-dopedcentral core region. Although such a profile utilizes the advantages ofthe MCVD process, it should be understood that the invention is not solimited. It may be applied to the production of simple un-doped corerods. Or it may be applied to making just a germanium up-doped core. Inmost cases, other profile regions of differing refractive index will beformed by one or more doped cladding tubes. Cladding tubes made withvery high quality glass-forming techniques may be used for most or evenall of the cladding layers. In the latter case, the MCVD process needsto supply only the central core region and standard single mode preformsequivalent to several thousand kilometers of fiber per preform meter canbe achieved.

After assembly of the rod 12 and tube 11, the tube is collapsed onto therod to produce the final preform 13, shown in FIG. 2, with the core rod14 indistinguishable from the cladding tube except for a smallrefractive index difference.

FIG. 3 represents a typical MCVD method. The starting tube is shown at31. An oxy-hydrogen torch 32 traverses the length of the outside of thetube while the tube is rotating. Glass precursor materials, typicallySiCl₄ and dopants such as GeCl₄, are introduced into the interior tubeat 33. When the glass precursors reach the hot zone, they form a sootdeposit 34 downstream of the torch on the tube wall as shown. This sootlayer is sintered into glass as the torch traverse moves the hot zonedownstream. Multiple passes form thicker glass deposits, and allow thecomposition of the glass deposit to vary radially from the tube center.The MCVD process is well known, and details of the process need noexposition here. See for example, J. B. MacChesney et al, “Preparationof Low Loss Optical Fibers Using Simultaneous Vapor Phase Deposition andFusion”, Xth Int. Congress on Glass, Kyoto, Japan (1973) 6-40.

According to the invention, most, and preferably all, of the centralcore material is deposited inside the MCVD tube. The ratio of thecentral core diameter to the preform diameter for a typical single modefiber preform is in the range 1/10 to 1/20. As the desired preform sizeincreases, the required central core size will necessarily increase. Inconventional MCVD, the MCVD starting tube limits the amount of depositedcentral core material to a relatively small fraction of the total amountof MCVD material. This is often expressed as the clad to core ratio orD/d ratio where D refers to the diameter of the total MCVD deposition(central core plus deposited cladding) and d refers to the diameter ofthe central core structure. Typical values of the D/d ratio for a simplesingle mode fiber design made by conventional MCVD with a commercialquality starting tube and designed to match a conventional overcladdingprocess are in the range of 2.0 to 4.0. In the method of the invention,there is no limit to the proportion of MCVD material that can be usedfor the core since the MCVD tube is not intended to be used in the finalpreform structure. That is to say, the invention allows the attainmentof the optimum D/d=1.0 ratio for a large preform. In this case, 100% ofthe MCVD material is used to form the central core. The cladding will beapplied later in the overcladding process, after removal of the startingtube, and each cladding can be any desired thickness where the finalpreform diameter is in proportion to the central core diameter. Thisallows multiple doped (or undoped) layers, of essentially any desiredthickness and sequence, to be incorporated in the preform design. Sincethe preform size scales as the inverse square of the D/d ratio, a cladto core ratio of 1.0 would provide a factor of 4 increase in preformsize compared to a conventional MCVD single mode fiber process with aD/d ratio of 2.0. While a D/d ratio of 1.0 corresponds to the largestpossible preform size for a given amount of MCVD deposition, in somecases it may be advantageous to deposit the central core and an innercladding region or regions by MCVD. In that case the clad to core ratiowill be greater than 1.0 but can still provide a significant advantagein comparison with standard MCVD practice. For example, if a centralcore and adjacent inner cladding with equivalent amounts of depositionare produced by MCVD, the clad to core ratio would be 2. Such a preformwould still be twice as large as a conventional MCVD preform with a cladto core ratio of 2.0, both preforms having the same total amount of MCVDdeposit.

Two typical preform profiles are shown in FIGS. 4 and 5. These areschematic plots and are general, since actual preform profiles are notpart of the invention. The Y-axis is the refractive index variation fromthat of un-doped silica (zero), with up-doping on the + part of thescale and down-doping on the—side of the scale. The x-axis is theposition along the radius of the preform with a typical amount of MCVDdeposition indicated schematically. In commercial practice, depositedglass diameters in the collapsed preform in the range of 12 to 14 mm arereadily achieved by MCVD. As is well known, the profile will beessentially replicated in the drawn fiber, but the plots shown are forthe preform. FIG. 4 is intended to show generally a profile with arelatively heavy up-doped core region 41, an un-doped (or neutrallydoped) inner cladding region 42, and an up-doped outer cladding region43. Regions 41 and 43 are typically doped with germanium. There areseveral options available for producing this profile using the method ofthe invention. The core rod may be made with just the core material(region 41) extending to the limit of MCVD deposition. At the otherextreme, the entire profile may be encompassed within the MCVDdeposition. However, as mentioned above, the latter results in preformsize limitations. Thus the invention is mainly directed to forming some,but fewer than all, of the core/cladding segments with MCVD. It is alsopossible to design the process for partial formation of a given claddingsegment using MCVD, with the remainder of that segment comprising acladding tube. In a preferred embodiment, the core and the firstcladding segment are made by MCVD, with the remaining segments formedusing one or more cladding tubes. For some complex profiles, as many asfive cladding tubes may be used.

For example, if the core 41 and the inner cladding 42 are made by MCVD,that would involve 12-14 mm (diameter) of MCVD material.

The profile in FIG. 4 is well known and has been widely used. Othervariations in profile design are represented generally by the profile inFIG. 5. Here the center core region 51 is relatively lightly doped withgermanium, yielding a high quality low loss core region. The innercladding layer, 52, is down-doped with F. The outer cladding layer 53 iseither un-doped, or up-doped slightly. Again, by substitutingappropriately doped overclad tubes for the outer cladding segment or theinner and outer cladding segments, the amount of MCVD core material willbe sufficient for a very large preform.

It will be understood by those skilled in the art that the two profilesshown in FIGS. 4 and 5 are just typical of a large number of profilevariations now known, or to be developed. These may have more than threecore/cladding segments. It will also be appreciated that the ability toform core material to an increased thickness, allows wide versatility incore design.

When deposition of the profile regions that are to be provided by MCVDis complete the tube is collapsed by known techniques, i.e. heating thetube to above the glass softening temperature, i.e. >2000-2400° C. toallow the surface tension of the glass tube to slowly shrink the tubediameter, finally resulting, after multiple passes of the torch, in thedesired solid rod. The collapsed rod is shown in FIG. 6, with the MCVDstarting tube shown at 61, and the MCVD deposited core (orcore/cladding) at 62.

Next, in accordance with a principle step of the invention, the MCVDtube is removed. This may be accomplished by mechanical grinding, byplasma etching, by chemical etching or by a combination of thesetechniques. In certain cases, depending on the quality of the startingtube material, it may be permissible to leave a residual amount ofstarting tube material surrounding the MCVD deposited glass but in apreferred embodiment, all the starting tube glass is removed. The endpoint of the etching process can be determined from a refractive indexprofile of the collapsed rod. If the desired profile is similar to thatof FIG. 5, with the first deposited layer 53 undoped, there is somemargin for error in the case where complete removal of the starting tubeis required. The etched preform may be measured after grinding oretching is complete to determine the amount of overetching, which isthen factored into the selection of the cladding tube. It will beevident that overetching is preferable to underetching in this case.Accordingly, the MCVD deposition and the etch time may be designed forlimited but finite etching of MCVD deposited material.

In general, at least 75% of the starting tube cross sectional area willbe removed in practicing the invention. This may be expressed as:((OD ₂)² −ID ²)/((OD ₁)² −ID ²)<0.25

where OD₁ and OD₂ are the outside diameters of the collapsed rod beforeand after removal, and ID is the inside diameter of the starting tubeafter collapse. Preferably, according to the invention, more than 90% ofthe tube is removed, and more typically, all of the tube is removed.

After removing the MCVD starting tube, the MCVD deposited glass coreremains, as shown in FIG. 7. Before overclad, the surface of the corerod may be cleaned by plasma treatment or by chemical etching to removeany residual OH⁻, and other contaminants. The finished rod may then beinserted in a cladding tube as described above.

As noted earlier, the MCVD process is limited in the total amount ofglass that can be deposited inside a starting tube. Typical commercialpractice, if directed towards single mode preforms, would result in lessthan 15 mm of total MCVD deposit (expressed as MCVD glass diameter inthe collapsed rod) although somewhat larger amounts can be achieved withspecial effort. If the intended size of the final preform is largeenough, a substantial fraction of that total MCVD deposited material isutilized to form the central core. In that case, significant opticalpower can extend in the drawn fiber outside the MCVD deposited regionand it will be advantageous to perform the overcladding process, or atleast the first overclad if a multiple overclad process is used, in away that assures the optical quality of the interface between the MCVDmaterial and the overclad tube and avoids the generation of a layer ofhigh loss glass. This can be accomplished by controlling the atmospherein the gap between the MCVD rod and the overcladding tube in much thesame way that the original MCVD process carries out the glass formingreaction inside a tube to isolate the glass forming reaction from theenvironment. In the overcladding case, the overcladding tube with theMCVD derived core rod inside can be attached to an MCVD lathe or similarapparatus and said overclad tube, coupled to the lathe gas deliverysystem, then provides the isolation form the ambient environment.

A suitable apparatus for conducting this step is illustrated in FIG. 8.where a cladding tube 71 is shown supported by lathe elements 72 and 73.Element 71 represents the tube support and exhaust structure, andelement 73 is a rotating seal supporting the tube on the inlet side. Thecore rod is shown at 74 with spacers 75 supporting the core rod withinthe cladding tube. The spacers are designed to allow gas flow along thetube. The standard MCVD type torch is shown schematically at 76. Theatmosphere 77, between the core rod 74 and the cladding tube 71 iscontrolled in this arrangement and the composition of the controlledatmosphere is determined by flowing gas from gas inlet 78.

A gas composition that is effective in removing hydrogen is introducedinto the gap 77 and one or several passes of the heat source 76 alongthe length of the tube can be made while the gas is flowing. Effectivegas ambients would include a drying agent such as chlorine or a dryingagent such as chlorine with an etching agent such as a chlorofluocarbonor sulphur hexafluoride to remove a very thin surface layer. Ahydrogen/oxygen torch (as schematically shown in FIG. 8) can serve asthe heat source but a preferred method is to use a hydrogen free heatsource such as a plasma torch or an electrically heated furnace. Duringthese traverses the heat source maintains the tube at a temperaturesufficient for effective drying of the interior surfaces but below thetemperature at which the tube would collapse onto the rod. After asufficient number of drying/etching traverses of the heat source, thetemperature is increased and the collapse is carried out in one or morepasses. After the overclad process is completed, it may be advantageousto remove any residual surface OH on the outside of the overclad preformwith either a plasma etch process, a chemical etch process or acombination of the two.

In addition to assuring the quality of at least the first overcladinterface, it may be useful to use an ultra-high-purity tube for thefirst overclad. Ultra-high-purity may, in a preferred case, be definedas having less than 50 ppB hydroxyl ion by weight. This is especiallybeneficial in the case where significant optical power is carried in thefiber in the region corresponding to this tube, and where a tube oftypical quality would introduce noticeable excess loss. As noted above,this tube, since it is only used in an overcladding process, may be of asize or dopant composition that typically would not be used as an MCVDstarting tube. Examples include very thin walled tubes, tubes downdopedin refractive index with high levels of fluorine, or tubes updoped inrefractive index with germanium. A more elaborate example can beenvisioned where an overcladding tube is fabricated with severaldistinct regions and each region has a different dopant profile toproduce a large preform version of a fiber design with multiple claddingstructures. Alternatively, the same goal could be approached by amultiple overclad process.

In some cases, it will be advantageous to introduce an elongation stepat an intermediate stage of the multiple overcladding sequence. In thiselongation step, the glass body, after the completion of one or moreoverclad steps, is stretched in length and reduced in diameter.Additional overcladding steps may be carried out after the stretching toproduce a final preform ready for draw. Inclusion of an elongation orstretching step does not change the fiber kilometer equivalent of theoriginal core rod but can allow a smaller diameter (and longer length)final preform which may be more suited to a particular fiber drawfacility.

As just described, the use of an ultra-high-purity first overclad tubeis also preferred if the fiber design has enough optical powerpropagating outside the MCVD region to otherwise adversely affect thefiber loss. The MCVD core glass will typically have a diameter of 12 to15 mm. The central core region that contains most of the guided lightmay have a diameter of 6 mm to 15 mm. The overall preform, afterapplying one or more cladding tubes, may have a diameter of 200 mm ormore. Thus the method of the invention provides for a very largepreform, with a complex profile structure, in which the core is formedentirely by MCVD. For the purpose of definition, a large preform isconsidered as one with a diameter of at least 120 mm.

Although in the description so far the MCVD core rod is intended as partof a rod-in-tube method, alternatively the MCVD core rod may be used asa substrate for soot deposition. In this way, cladding layers or partialcladding layers may be deposited using soot techniques.

Although the MCVD process as described above uses a flame torch and afuel of mixed oxygen and hydrogen, plasma torches or electrically heatedfurnaces may also be used in these kinds of processes. Also, gas torchesother than oxy-hydrogen torches can be used.

The optical fiber preform, as described above, is then used for drawingoptical fiber. FIG. 9 shows an optical fiber drawing apparatus withpreform 81, and susceptor 82 representing the furnace (not shown) usedto soften the glass preform and initiate fiber draw. The drawn fiber isshown at 83. The nascent fiber surface is then passed through a coatingcup, indicated generally at 84, which has chamber 85 containing acoating prepolymer 86. The liquid coated fiber from the coating chamberexits through die 91. The combination of die 91 and the fluid dynamicsof the prepolymer, controls the coating thickness. The prepolymer coatedfiber 94 is then exposed to UV lamps 95 to cure the prepolymer andcomplete the coating process. Other curing radiation may be used whereappropriate. The fiber, with the coating cured, is then taken up bytake-up reel 97. The take-up reel controls the draw speed of the fiber.Draw speeds in the range typically of 1-20 m/sec. can be used. It isimportant that the fiber be centered within the coating cup, andparticularly within the exit die 91, to maintain concentricity of thefiber and coating. A commercial apparatus typically has pulleys thatcontrol the alignment of the fiber. Hydrodynamic pressure in the dieitself aids in centering the fiber. A stepper motor, controlled by amicro-step indexer (not shown), controls the take-up reel.

Coating materials for optical fibers are typically urethanes, acrylates,or urethane-acrylates, with a UV photoinitiator added. The apparatus isFIG. 9 is shown with a single coating cup, but dual coating apparatuswith dual coating cups are commonly used. In dual coated fibers, typicalprimary or inner coating materials are soft, low modulus materials suchas silicone, hot melt wax, or any of a number of polymer materialshaving a relatively low modulus. The usual materials for the second orouter coating are high modulus polymers, typically urethanes oracrylics. In commercial practice both materials may be low and highmodulus acrylates. The coating thickness typically ranges from 150-300μm in diameter, with approximately 240 μm standard.

As an example of the described process, we have fabricated a preformsized to yield 1500 kilometers of fiber per meter of core rod length. Arefractive index profile obtained from a sample of the drawn fiber isshown in Figure X. The graded central core region and the adjacentdowndoped inner cladding region were fabricated with MCVD inside anundoped silica starting tube. As described above, after the MCVDcollapse step the starting tube was completely removed by a processcombining mechanical grinding, plasma etching and chemical etchingleaving a core rod of MCVD material approximately 13 mm in diameter.This MCVD glass rod was then overclad with an ultra-pure germanium dopedsilica overclad tube which provided the updoped cladding region seen inthe refractive index profile. This first overclad operation was carriedout as described earlier with care to preserve the optical quality ofthe glass in the interface region. Three more overclad steps, one ofwhich made use of a downdoped overclad tube, along with an intermediatestretching step to size the final preform diameter for a conventionalfiber drawing furnace, were used to complete the preform. The opticalproperties of the fiber drawn from this preform were equivalent to thoseof fibers with similar refractive index profiles drawn fromsignificantly smaller preforms made with a conventional, prior art MCVDand overclad process. In particular, the optical loss of the fiber drawnfrom the example preform was equivalent or lower than was typical offiber drawn from such prior art preforms. To illustrate this point, FIG.11 shows a spectral loss curve (fiber attenuation vs. wavelength) of afiber from the example preform.

Various additional modifications of this invention will occur to thoseskilled in the art. All deviations from the specific teachings of thisspecification that basically rely on the principles and theirequivalents through which the art has been advanced are properlyconsidered within the scope of the invention as described and claimed.

1. Process for the manufacture of optical fiber comprising: (a)preparing an optical fiber preform, (b) heating the preform to thesoftening temperature, and (c) drawing an optical fiber from thepreform, the invention characterized in that the optical fiber preformis prepared by steps comprising: (i) forming by MCVD a first glass layeron the inside of a MCVD starting tube and a second glass layer on thefirst glass layer, the MCVD starting tube comprising a first glassmaterial, and where the first glass layer is a core layer having a firstrefractive index and the second glass layer is a first cladding layerhaving a second refractive index lower than the first refractive index,(ii) collapsing the MCVD tube to produce a first solid glass cylindricalbody, (iii) removing at least a portion of the first glass materialleaving a second solid glass cylindrical body of the MCVD glassmaterial, and (iv) applying a cladding to the second solid glasscylindrical body of MCVD glass material by inserting the second solidglass cylindrical body of MCVD glass into a cladding tube and collapsingthe cladding tube around the second solid glass cylindrical body of MCVDglass material.
 2. The process of claim 1 wherein the MCVD starting tubehas an outside diameter OD, and an inside diameter ID and the portion ofthe first glass material of the MCVD starting tube cross sectional areathat is removed is:((OD ₂)² −ID ₂)/((OD ₁)² −ID ²)<0−0.25 where OD₂ is the outside diameterof the second solid glass cylindrical body.
 3. The process of claim 2wherein all of the first glass material is removed.
 4. The process ofclaim 1 wherein the step of applying a cladding involves mounting thesolid glass cylindrical body within the overclad tube leaving an ambientspace between the solid glass cylindrical body and the overlad tube, andcontrolling the ambient space by flowing gas through the ambient space.5. The process of claim 1 wherein the overclad tube comprises glass witha hydroxyl ion content less than 50 ppB by weight.
 6. The process ofclaim 1 wherein said overclad tube is up-doped with germanium.
 7. Theprocess of claim 1 wherein said overclad tube is downdoped withfluorine.
 8. The process of claim 1 where the solid glass cylindricalbody has a diameter of at least 12 mm.
 9. The process of claim 1 whereinthe first glass material is removed by mechanical grinding.
 10. Theprocess of claim 1 wherein the first glass material is removed by plasmaetching.
 11. The process of claim 1 wherein the first glass material isremoved by chemical etching.
 12. The process of claim 1 wherein thefirst glass material is removed by a combination of methods includingmechanical grinding, plasma etching and chemical etching.
 13. Processfor the manufacture of an optical fiber preform comprising: (a) formingby MCVD a first glass layer on the inside of a MCVD starting tube and asecond glass layer on the first glass layer, the MCVD starting tubecomprising a first glass material, and where the first glass layer is acore layer having a first refractive index and the second glass layer isa first cladding layer having a second refractive index lower than thefirst refractive index, (b) collapsing the MCVD tube to produce a firstsolid glass cylindrical body, (c) removing at least a portion of thefirst glass material leaving a second solid glass cylindrical body ofthe MCVD glass material, and (d) applying a cladding to the second solidglass cylindrical body of MCVD glass material by inserting the secondsolid glass cylindrical body of MCVD glass into a cladding tube andcollapsing the cladding tube around the second solid glass cylindricalbody.
 14. The process of claim 13 wherein the MCVD starting tube has anoutside diameter OD, and an inside diameter ID and the portion of thefirst glass material of the MCVD starting tube cross sectional area thatis removed is: ((OD₂)²−ID₂)/((OD₁)²−ID²)<0−0.25 where OD₂ is the outsidediameter of the second solid glass cylindrical body.
 15. The process ofclaim 13 wherein all of the first glass material is removed.
 16. Theprocess of claim 13 wherein the step of applying a cladding involvesmounting the second solid glass cylindrical body within the overcladtube leaving an ambient space between the solid glass cylindrical bodyand the overclad tube, and controlling the ambient space by flowing gasthrough the ambient space.
 17. The process of claim 16 wherein theoverclad tube comprises glass with a hydroxyl ion content less than 50ppB by weight.
 18. The process of claim 13 wherein the first glassmaterial is removed by a combination of methods including mechanicalgrinding, plasma etching and chemical etching.